Multiplex systems for sterophonic transmission or recording are well known, such systems using a subcarrier for transmission of certain information in an inaudible part of the spectrum are widely in use. They have several advantages among which the most important is the fact that the stero signal is compatible with monaural reproducing equipment because monaural reproduction of the primary information is always possible without the need for special equipment in the receiver or reproducer. A decoding device responsive to the subcarrier signal need be provided only if the improvement is to be employed in sterophonic reproduction of frequency modulated signals or in quadraphonic listening to disc-recordings so recorded.
This compatibility is achieved in two channel stero systems by including in the multiplex signal a "sum" or main signal (left plus right) and a subcarrier modulated by a "difference" or composite signal (right minus left). Generally speaking, the main signal is a signal directly usable in monaural devices and is generally transmitted just as it is, while the composite signal is a signal resulting in non-directly usable information and is generally transmitted by modulation of a subcarrier.
Although these systems are satisfactory in theory, in practice they are accompanied by an important degradation of the signal-to-noise ratio when the subcarrier signal is used to carry the composite signal. The reason for this degradation is essentially that the part of the spectrum occupied by the modulated subcarrier is reproduced in the audio spectrum with its own noise-content; this downgrading of the signal-to-noise ratio being in the vicinity of 25 dB in weighted value in FM recption when changing from monaural to stereophonic listening.
In practice, this phenomenon expresses itself by a decrease in the usable range of the stero FM transmitters. Sophisticated up-to-date receivers, mainly vehicle receivers, are provided with an automatic switching from stereo to monaural listening when the conditions for reception become unfavorable for stereo listening.
In order to reduce this defect, it has been proposed to use a noise-reduction process during transmission. This process not being a compatible one will penalize the users of existing receivers which are not adapted to correct the signals so processed. Of course, future receivers can be provided with the necessary decoding circuits. Hence there is a need for a compatible system that will permit an improvement of the signal-to-noise ratio of stereophonic signals, primarily in the case of FM reception.
The fact that such a significant downgrading of the S/N ratio occurs when changing from monaural to stero listening demonstrates that substantial noise is entirely introduced by the composite "difference" signal in the case of FM reception, for example. Accordingly, a noise reduction treatment made on this signal only will bring a noticeable improvement. What is more, inasmuch as the main "sum" signal is undisturbed by the noise reduction treatment of the composite signal and carries all of the audio frequency components of the signal, it is possible to enhance the treatment of the composite "difference" signal up to a point of some deterioration that will in fact be indistinguishable during listening because it is masked by the presence of the main "sum" signal.